Superparamagnetism and giant magnetoresistance in sputtered FeCuAg granular films

Magnetization and magnetoresistance studies were carried out on the granular FexCuyAgz films prepared by dc magnetron sputtering. In this series of samples, we have substituted Ag in the parent Fe–Cu system such that the Ag concentration increases from zero while that of the Cu decreases to zero when we go from one sample to another. The blocking temperatures of the films lie in the range 20–50 K. By using a model, the room temperature magnetization data have been analyzed by determining the contributions from the smaller superparamagnetic particles and larger ferromagnetic particles. Size distributions of the superparamagnetic particles are obtained. The magnetoresistance and (M/Ms)2 data do not agree for most of the samples investigated. This discrepancy is due to the fact that the (M/Ms)2 behavior is valid for purely superparamagnetic particles of only one size whereas there are both superparamagnetic and ferromagnetic particles in our samples. When we analyzed the magnetoresistance data using a superparamagnetic model, a good agreement between the experimental data and the simulated data were obtained for some samples. Significant discrepancy observed for other samples may be due to the spin dependent scattering of the electrons traveling from superparamagnetic to ferromagnetic particles or vice versa.